659

class Solution {
public:
    bool isPossible(vector<int>& nums) {
        deque<int> len;
        vector<pair<int, int>> count;
        int n=nums.size();

        // Have a list of val-count pair.
        for(int i=0; i<n; i++){
            int curVal = nums[i], cnt=0;
            while(i<n && nums[i] == curVal){
                cnt++, i++;
            }
            i--;
            count.push_back({curVal, cnt});
        }

        // Start iteration over val-count pair.
        bool possible = true;
        for(int i=0; i<count.size() && possible; i++){
            // If curVal != prevVal+1
            if(i>0 && count[i].first != count[i-1].first+1){
                while(!len.empty()){
                    int curLen = len.front();
                    len.pop_front();
                    if(curLen < 3){
                        possible = false;
                        break;
                    }
                }
            } // If count is greater-equal than deque length.
            else if(count[i].second >= len.size()){
                int needed = count[i].second - len.size();
                while(needed--)
                    len.push_back(0);
                for(auto it=len.begin(); it!=len.end(); it++)
                    (*it)++;
            } // If count is less than deque length.
            else if(count[i].second < len.size()){
                int toRemove = len.size() - count[i].second;
                while(toRemove--){
                    int curLen = len.front();
                    len.pop_front();
                    if(curLen < 3){
                        possible = false;
                        break;
                    }
                }
                for(auto it=len.begin(); it!=len.end(); it++)
                    (*it)++;
            }
        }

        // Empty the deque.
        while(!len.empty()){
            int curLen = len.front();
            len.pop_front();
            if(curLen < 3){
                possible = false;
                break;
            }
        }
        return possible;
    }
};